This invention relates to an improved window for a display device, and associated display assembly. The invention has a range of possible uses in handheld or portable devices such as portable telephones, remote control units, laptop computers or personal audio equipment, as well as in larger fixed items such as microwave ovens and industrial machines.
Liquid Crystal Displays (LCDs) are well known display devices and are used for indicating status information in a wide range of everyday devices. Unlike some other display devices such as Light Emitting Diodes (LEDs) or Cathode Ray Tubes (CRTs), they do not inherently emit any light themselves.
LCD displays, therefore, are either only usable in situations where there is sufficient incident light falling upon them, or else where the device is equipped with suitable lighting facilities of its own.
One form of lighting is backlighting, which can only be used with certain types of LCDs. Backlighting, though, can be quite inefficient due to the number of layers making up the display through which the light has to pass. For instance, a typical LCD display comprises at least: two layers of glass, two layers of polarising film, two layers of electrodes and the actual liquid crystals themselves. The polarising films can account for a loss of more than 50% of the transmitted light. In total, a monochrome LCD may only transmit 10% of the light emitted from the backlight. The situation becomes even worse for colour LCDs which may transmit less than 5% of the light emitted from the backlight. A typical LCD 10 is shown at FIG. 1. The structure of the LCD is seen to consist of several layers. The outermost layer, and the side from which the display is viewed, is the polarising layer 15. Immediately beneath that is a layer of glass 20. Disposed on the surface of the glass layer 20 is electrode layer 25. Situated between the electrode layer 25 and a further electrode layer 35 is a layer of liquid crystal material 30. The liquid crystal material is retained in position between glass layers 20 and 40 by seal 50. The final layer is a further polariser 45.
The operation of LCDs will be well known to the skilled man, and so will not be described in any detail here.
There are three basic modes of operation for LCD displays: reflective mode, transmissive mode and transflective mode.
In reflective mode LCDs, the lower layer consisting of the polariser 45 is effectively mirrored. This means that any light entering from the front of the display 15 is reflected outwards again. Such displays tend to offer the best brightness and contrast. Backlighting of such displays is not possible due to the reflective layer, which will not transmit light.
In transmissive mode LCDs, the polariser layer 45 has no reflective properties. For that reason, for the LCD display to display any information at all, a permanent backlight must be provided. This, of course, can place a considerable drain on battery-powered equipment, and so is not suitable for devices such as portable telephones.
Transflective mode LCDs operate midway between reflective and transmissive displays. The polariser layer 45 is provided with a coating providing partial reflectivity, and partial transmissivity. In situations where there is sufficient ambient light, the display is visible by that light alone. However, when the ambient light level is low, backlighting can be used to enable the displayed information to be visible.
With the advent of colour LCDs, the lighting problem becomes more pronounced. The presence of additional coloured filters into the layers shown in FIG. 1 further reduce the light passed by the display.
With colour displays, it is particularly desirable to be able to maintain a good display brightness and contrast. This is best achieved using a reflective mode display. However, such displays, as explained above, are not usable with backlighting, limiting their application in certain products, where good ambient lighting may not always be available.
It is possible to provide additional front-lighting for such displays, but this introduces further problems, such as ensuring the display is not obscured by the light sources.
A prior art technique of front lighting includes the structure as shown in FIG. 2, which shows a cross section through a display assembly 100. Display assembly 100 has the following parts: a display window 130; a light guide, or grating 120; an LCD component 110; and a light source 140.
The LCD component 110 operates in reflective mode and is a complete unit which may be obtained from a number of suppliers such as Sharp, Philips or Seiko. The LCD has electrical connections for connection to a processor for controlling the information displayable thereon. The LCD 110 may be of the order of several millimetres thick
Disposed between the display window 130 and LCD 110 is a light guide or diffusion grating 120. This may be approximately 1 mm thick. The light guide 120 has the function of distributing the light from light source 140 and so illuminating the display from its front surface. Light Source 140 may be any convenient source such as an LED.
Disposed above the grating 120, and having its outer surface exposed to the user, the display window 130 is a substantially transparent plastics window, which may be approximately 1 mm thick. This may be treated on its exposed outer surface with a scratch resistant coating and/or an anti-reflective coating.
The purpose of the display window is to provide protection for the delicate structures of the grating 120 and LCD 110, while still allowing the LCD 110 to be visible through the window.
The display window 130, grating 120 and LCD 110 are separated from each other by a small distance, which may of the order of 0.1 to 0.5 mm by use of a gasket arranged between each layer and around the edge of each layer. The gasket may be made of a plastics or rubber material.
The separation between the various elements is to provide a degree of protection to them in case of the display assembly being knocked. The LCD 110 comprises glass, and is very delicate. The grating 110, having a very fine surface, can be easily damaged by any contact with other bodies. The display window is more rugged than the other components of the display and serves to protect them from inadvertent damage.
With many layers making up a display assembly, there is the possibility of internal reflections between the various layers causing display degradation. Such degradation may take the form of blurring of the image, and loss of contrast. In order to minimise the problem of internal reflections between the various layers, anti-reflective coatings may be added to some or all of the components of the assembly. The addition of such coatings adds to the cost of the display assembly.
The grating 120 is made from a plastics material, and has etched onto its surface an array of so-called microgrooves. The grooves may be formed by any conventional method. Suitable methods are acid etching, laser etching, moulding and machine-removal of excess material.
The function of the microgrooves is to distribute the light coming from a light source 140 disposed to one or more sides of the grating. The microgrooves distribute the light downwards through approximately 90° so that it illuminates the LCD situated below. The light source is positioned substantially level with the light guide 120 as shown in FIG. 2.
The operation of the microgrooves is shown in FIGS. 3a and 3b. FIG. 3a shows a plan view of the light source and the grating, and FIG. 3b shows a cross-sectional view of the grating 120, light source 140 and LCD 100. The light source 140 is arranged to shine across the width of the grating 120 from the side of the grating as shown. When light from the light source hits each microgroove 125, it is reflected downwards towards the LCD 110 through the body of the light guide 120. The dimensions of each component have been greatly exaggerated for clarity, and only the operation of 3 microgrooves is illustrated. The microgrooves are in fact only a few microns wide, and run generally parallel in one direction across substantially the entire length of the grating.
Depending on the size of the display and luminosity of the light source, among other factors, more than one light source may be provided. For instance, in a monochrome mobile telephone display measuring 4 cm by 3 cm, two light sources may be required.